The present invention relates to aqueous emulsion polymerized latexes having a predominant proportion of sec-butyl acrylate suitable for use as pressure sensitive adhesives and to articles comprising surfaces coated with such pressure sensitive adhesive polymers.
As used in the art, the term "pressure sensitive adhesive" designates a polymeric material which in solvent-free form is aggressively and permanently tacky at room temperature, and will firmly adhere to a variety of dissimilar surfaces upon mere contact without the need of more than finger or hand pressure. It is generally desirable that, in order that the pressure sensitive adhesive adhere well, the critical surface energy of the polymeric material be as low as possible. Typically, low critical surface energies are associated with highly nonpolar polymers.
Pressure sensitive adhesives are conventionally used in the form of films which provide adhesive bonds between normally nonadhering surfaces. The films have been prepared by casting from organic solvent solutions of pressure sensitive interpolymers and evaporating the solvent. A disadvantage of such organic solutions arises from large quantities of organic solvent effluent which is discharged into the atmosphere. Manufacturers of pressure sensitive articles are, therefore, switching increasingly from organic solutions to aqueous emulsions of pressure sensitive interpolymers to minimize atmospheric pollution. Aqueous emulsions of essentially water-insoluble polymers are applied to surfaces to yield pressure sensitive adhesive coatings.
The most important properties recognized by the pressure sensitive adhesive industry are adhesion or peel strength; "tack"; and resistance to shear at elevated temperatures. The addition of a tackifier can improve the peel strength and adhesion exhibited by a pressure sensitive adhesive at room temperature. Unfortunately, at higher temperatures most tackifiers serve as plasticizers and promote shear failures.
A balance of desirable properties can be achieved by preparing a copolymer comprising varying amounts of "soft" monomer (i.e., having a glass transition temperature below 25.degree. C.) and "hard" monomer (i.e., having a glass transition temperature above 25.degree. C.). For example, large amounts of "soft" monomers such as butadiene, i-butyl acrylate, n-butyl acrylate, t-butyl acrylate or 2-ethylhexyl acrylate in a copolymer improve the adhesive strengths of the resulting pressure sensitive adhesives, but at the expense of thermal shear endurance. Conversely, the polymers comprising large amounts of "hard" monomers will exhibit good thermal shear endurance at the expense of tack and adhesive properties.
The use of alkyl acrylic esters in preparing polymers and copolymers which are then employed as pressure sensitive adhesives is disclosed in U.S. Pat. No. 3,691,140. However, U.S. Pat. No. 3,691,140 only discloses acrylic ester polymers prepared by aqueous suspension polymerization as being suitable for tacky, infusible microspheres. It has not been documented that sec-butyl acrylate has superior properties when employed in preparing pressure sensitive adhesives with respect to other alkyl acrylic esters.
In view of the fact that the pressure sensitive adhesives which have been documented to date have been prepared through balancing properties such as cohesion, adhesion and high temperature shear resistance, it would be highly desirable to provide a polymer which can be employed as an aqueous emulsion which has a low critical surface energy and which exhibits extremely high cohesion and adhesion without the loss of high temperature shear resistance.